Hydraulic jars

ABSTRACT

A HYDRAULIC JAR FOR APPLYING A JARRING BLOW TO A STUCK PIPE OR FISH IN A WELL, WHEREIN MEANS ARE PROVIDED FOR PERMITTING RAPID AND SUBSTANTIALLY UNRESTRAINED CLOSING OR RETURN OF THE JAR TO THE PULLING POSITION, AND WHEREIN MEANS ARE PROVIDED FOR DEVELOPING GREATER PULLING FORCES   THAN WITH PRIOR HYDRAULIC JARS WITHOUT BECOMING JAMMED OR STUCK INTERNALLY DURING THE PULLING STROKE, EVEN IN WELLS HAVING HIGH PRESSURES.

Feb. 9, 1971y 0,1', SLATOR ETAL n 3,562,807

HYDRAULIC JARSv y Filed sept. ,20, 1968 4 5 sheets-sheet 1 Feb. 9, 1971 D, T SLATOR ET 'AL 3,562,807

HYDRAULIC J ARS 3 Sheets-Sheet 2 Filed Sept. 20. 1968 AN/X /w V A/ am* mw mNl mw\ W m 2% 3pm rrmm mm www wA ATTORNE YS Feb, 9, 1971 D. T. sLAToR ETAL 3,562,807

HYDRAULIC J ARS Filed Sept. 20, 1968 3 SheelS-SIleeiI 3 .czmon 7.' J/aof- W////am 7T L ee Af'c/e W Pev/ INI/ENTORJ' #naden phone! WIM, f MaNLewA /ITTORNE YS Patented Feb. 9, 1971 3,562,807 HYDRAULIC JARS Damon T. Slator, Houston, William T. Lee, Magnolia, and Archie W. Peil, Houston, Tex., assignors to Bowen Tools, Inc., a corporation of Texas Filed Sept. 20, 1968, Ser. No. 767,905 Int. Cl. E21b 1/06 U.S. Cl. 175-297 9 Claims ABSTRACT OF THE DISCLOSURE A hydraulic jar for applying a jarring blow to a stuck pipe or fish in a well, wherein means are provided for permitting rapid and substantially unrestrained closing or return of the jar to the pulling position, and wherein means are provided for developing greater pulling forces than with prior hydraulic jars without becoming jammed or stuck internally during the pulling stroke, even in wells having high pressures.

BACKGROUND OF THE INVENTION The field of this invention is hydraulic jars for applying a jarring blow to stuck pipe or `fish in a well.

In the past, hydrauic jars have been constructed with a piston which has been pulled or pushed through a restriction and then suddenly released to produce a jarring blow on the pipe or stuck fish to which the jar is attached. See, for example, U.S. reissue patent Re. 23,354 dated Apr. 10, 1951 granted to Lynn W. Storm. These jars have been made as two-way jars for jarring in either direction, and as one way jars for jarring in one direction only, usually on the upstroke. With one-way jars, ordinarily springs such as shown in FIG. of said Storm reissue patent Re. 23,354 have been employed to permit a comparatively rapid fluid by-pass when returning a one-way jar to the pulling position. Such springs and similar mechanism are subject to deterioration and malfunction, particularly in hard field usage such as encountered with this type of tool.

Additionally, such prior jars have employed various means for restricting the flow of fluid past the piston, such as leather, rubber, and Teflon, packings, and close fitting metal pistons having close or even negative clearance between the piston and the cylinder. All of such means are subject to sticking or jamming in the cylinder when pulling with the jar, especially when the pressure of the well uid is high as in deep wells where such fluid pressures are up to 20,000 p.s.i. and sometimes higher. External `well pressures exert a compressive force on the tool body which squeezes the body against the piston and causes such sticking or jamming of the piston in the body, rendering the jar action erratic, or even inoperable, during the pull stroke. Additionally, the closing or return of the jar to the pulling position has been restricted or prevented by the close or negative clearance of such prior tools. In the past, efforts to overcome such problems have included the use of an additional floating piston so the fluid pressure in the tool is equalized with the pressure in the Well prior to the pulling stroke; but, during the pulling stroke, the internal pressure is increased above the well pressure in an amount proportional to the pulling force on the tool, and this pressure differential acting on the body of the tool is a limiting factor with respect to the bursting of the body, regardless of the depth of the well in which the tool is used.

SUMMARY OF THE INVENTION With the present invention, the problem of the sticking or jamming of the piston in the jar has been solved by providing a unique piston assembly wherein the piston has a flexible metal-to-metal seal with the cylinder and a xed orice forming the restriction in the tool, and with the internal hydraulic area sealed off so that the fluld pressure in the Well is not additive to the pressure created in the tool by the pulling action with the jar as in the prior constructions with the floating pistons, but instead, the well pressure actually counterbalances some or all of the internal pressure developed in the tool during the pull stroke, so that in the presence of high well pressures, the jarring load can be increased without bursting the tool. Additionally, the present invention preferably provides an unrestrained or substantially unrestrained closing or return of the tool to the pulling position, even though the tool is unbalanced. No springs or similar objectionable mechanisms for the bypassing of fluid around the piston during the return stroke are utilized.

BRIEF 'DESCRIPTION OF TH-E DRAWING FIG. l is a diagrammatic view of the hydraulic jar of this invention in its telescoped or down position;

FIG. 2 is a view similar to FIG. 1, but illustrating the hydraulic jar in its uppermost or jarring position;

IFIG. 3 is an enlarged fragmentary Vertical sectional view illustrating the improved piston assembly of the hydraulic jar of this invention, with the pistons thereof in the raised position for bypassing fluid around the piston during the return stroke of the jar;

FIG. 4 is a view similar to FIG. 3, by illustrating the piston assembly during the pull or jarring stroke;

FIG. 5 is a cross-sectional view taken on line 5 5 of FIG. 3 to further illustrate the piston assembly of this invention;

FIG. 6 is a fragmentary sectional view of a modified form of the piston assembly during the return stroke of the jar;

FIG. 7 is a view similar to FIG. 6, except showing parts in elevation, during the pull or jarring stroke; and

FIG. 8 is a horizontal sectional View taken on line 8-8 of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS `In the drawings, the letter J designates generally the hydraulic jar of the present invention, the major portion of which is illustrated schematically in FIGS. 1 and 2, while the piston assembly P thereof is illustrated in detail in the drawings. The jar J includes an inner tubular body 10 which is of conventional construction, with the exception of the portion for mounting the piston assembly P, as will be explained. The inner tubular body 10 may be formed in one or more sections which are threaded or otherwise coupled together. The upper end 10a of the body 10 is threaded or is otherwise coupled to a pipe string (not shown) or other operating support which normally extends downwardly into a Well for the lowering, raising, and operation of the jar I. An outer tubular pipe 12, also of conventional construction, is disposed around the inner tubular body 10 so that the two bodies I0 and 12 may telescope with respect to each other as shown in FIG. 1 and so that they may move longitudinally relative to each other during the jarring stroke as illustrated in lFIG. 2. The lower end 12a of the outer tubular body 12 is connected by any suitable known grapple or the like to a struck pipe or ,fish in a well prior to performing the jarring operation with the jar J.

IA fluid or hydraulic chamber 14 is provided in the annulus between the tubular bodies 10 and 12, the extent of which is defined by an upper seal 15 and a lower seal 16. The seals 15 and 16 are preferably formed with rubber and/or Teflon and preferably incorporate O-rings for the sealing of the oil or other uid within the chamber 14 and so as to isolate such fluid from external well fluids.

The inner tubular body 10 has an annular hammer or jarring surface 10b which is adapted to hit an anvil or jarring surface 12b on the outer tubular body 12 when the bodies 10 and 12 are longitudinally extended with respect to each other during the jarring stroke, as will be more fully explained.

The piston assembly P is disposed within the uid in the hydraulic chamber 14 between the upper seal 15 and the lower seal 16, and, as will be explained, the piston assembly P serves to provide a restraint during the upward pulling on the inner tubular body 10 relative to the outer tubular body 12 so that tension and stretch develop in the pipe or tubing string connected to the inner tubular pipe 10. Such restraint is suddenly released as the piston assembly P moves into an enlarged portion 12C of the bore of the tubular body 12. The sudden release of the restraint on the inner tubular body 10 caused by the piston assembly P moving through the oil in the chamber 14, as will be explained, permits the stretch in the pipe to which the inner tubular body 10 is connected to exert a rapid and sharp upward movement bringing the lower jarring surface 10b into violet jarring contact with the jarring surface 12b to thus impart a jarring force to the sh or stuck object connected to the lower end of the tubular body 12.

Also, as will be explained, after the jarring stroke, the pipe supporting the tubular body 10 and the tubular body 10 itself are lowered relative to the outer body 12 and the piston assembly P is so constructed that there is substantially no restraint or restriction during such downward movement to return the piston assembly P to a position for again conducting a jarring stroke.

It should be noted that the piston assembly P is returned to the reduced diameter portion or restricted bore 12d in the tubular body 12, and the piston assembly P goes to the lowermost position even in such restricted bore 12d with substantially no uid restraint, as will be explained hereinafter.

The details of the piston assembly P of this invention are illustrated in FIGS. 3-5, wherein an annular piston 20 is illustrated as mounted on a by-pass body 22 for limited longitudinal movement relative thereto. The by-pass body 22 is secured to the inner tubular body 10 by any suitable means such as by disposing same between annular threaded nuts or sleeves 25 and 26. A seal body 27 which is annular in shape is preferably disposed between the lower ends of the by-pass body 22 and the threaded sleeve 26, although the body 22 and the seal 27 could be integral in some situations. It is to be noted that the seal body has an O-ring 28 or other suitable seal therewith to prevent fluid from flowing downwardly below the by-pass body 22 on the inside thereof. However, the outer diameter 27a of the seal body 27 is of a smaller diameter than the inside diameter f the restricted bore 12d so that there is an annular space through which the uids may pass without substantial restraint during the pulling and return stroke of the jar I, as will be more evident hereinafter.

The by-pass body 22 is provided with a plurality of longitudinal channels 22a (FIGS. 3-5), which together provide for the passage of sucient volume of fluid from one side of the piston assembly P to the other so that there is no restraint on the movement of the tubular body with respect to the tubular body 12 when such openings or channels 22a are in the open position.

The by-pass body 22 has an upper stop surface or shoulder 22b, which limits the upward travel of the piston with respect to the body 22 by the engagement of an inner laterally extending annular shoulder 20a with such stop shoulder or surface 22b.

The piston 20 has an upper outwardly flared exible lip 30 which has an outer cylindrical surface 30a which has a diameter equal to, or slightly larger than, the inner diameter of the restriction or restricted bore 12d. The surface 30a is a metallic surface, such as tool steel, and it is machined to a very high finish and to a close tolerance with respect to the inner diameter of the restricted bore 12d which is of a metal such as highly polished chrome plated steel. The lower portion of the external surface designated 20b is tapered inwardly so that there is substantial clearance between the surface 20b and the inside surface of the restricted diameter portion 12d. This minimizes the frictional contact between the piston 20 and the restricted diameter surface 12d, while maintaining a metal-to-metal seal therebetween. By reason of the ilexibility of the lip 30, the lip 30 maintains its non-binding and non-jamming seal with the surface of the restricted bore 12d during the pulling stroke of the piston 20 upwardly in the body 12. Thus, as the body 12 expands outwardly during the pulling stroke due to the increasing internal fluid pressurev in the chamber 14, the flexible lip 30 flexes outwardly also, whereby the sliding seal is maintained. Conversely, when there are high external Well pressures squeezing the body 12 inwardly, and the internal pressure is zero or lower than the external pressure, the lip 30 flexes inwardly as the body 12 is squeezed inwardly so that binding between does not occur; such a situation would occur, for example, when the piston 20 is on the return stroke and the lip 30 is moving downwardly in the restricted bore 20d.

The piston 20 has a lower annular edge or bottom surface 20c; one or more restricted passages 20d are preferably provided along said lower edge 20c so that uid may pass through such passage 20d when the piston 20 is in its lowermost position (FIGS. 1 and 4). The piston 20 is in such lower position during the pulling Stroke while the piston 20 is being pulled upwardly in the restricted diameter portion 12d. By forming the passage 20d as a semicylindrical notch or groove in the bottom surface 20c of the piston 20, foreign matter such as small particles of metal, rubber, or the like are Washed away from such passage 20d, particularly on the downstroke such as illustrated in FIG. 3 so as to prevent such passage 20d from becoming blocked or plugged.

In some situations, other types of restricted passages through or in the piston 20 may be provided. For example, the piston 20 may be formed as a split body with a longitudinal groove extending longitudinally therethrough. In such case, the piston 20 may still be considered to be annular, since the restricted longitudinal slit or passage serves merely as the restriction for obtaining the restraint previously described during the pulling stroke.

It should be noted that the lower surface 20c of the piston 20 is preferably polished metal which has a smooth surface to provide a metal-to-metal seal with an upper annular smooth metal surface 27b on the seal ring 27 when the piston 20 is in the lowermost position (FIG. 4).

It should be noted, also, that the upper end 30b of the lip 30 is spaced downwardly and outwardly from the bypass body 22 when the piston 20 has reached its uppermost position relative to the body 22 by the engagement of the shoulder 20a with the stop 22b. This assures the unrestricted flow of the oil or other liquid within the chamber 14 through the bypass around the piston 20 during the downstroke or return of the jar J for the subsequent jarring stroke.

In the operation or use of the jar of the invention, the lower end 12a of the outer tubular body 12 is connected to a stuck pipe or sh in a well. The upper end 10a of the inner tubular body 10 is connected to a pipe string or support extending to the surface of the well so that the operator may manipulate the jar J from the surface. During the running in of the jar J, it is normally in the position illustrated in FIG. 2. After the lower end 12a is connected to the stuck iish, if the jar J has not already been collapsed or moved to its lowermost position illustrated in FIG. l, this is then accomplished by lowering the pipe string and/or slacking it off so that the piston assembly P moves downwardly into the restricted bore or reduced diameter portion 12d. It is to be noted that such downward movement is accomplished |without any restraint from the iluid within the chamber 14 because the piston 20 upon entering the upper end of the restricted bore 12d drags its flexible annular lip 30 and is caused to be moved to its uppermost position (FIG. 3) as it moves downwardly within the restricted bore 12d. Thus, during the downward travel of the piston assembly P within the restricted bore 12d, the iluid below the piston assembly P and above the Vseal 16 moves upwardly around the piston 20 and through the bypass channels 22a without any appreciable restraint so that it may be said to be free closing. Also, there is a selfcleaning action provided by the construction of this invention since the channels 22a are fully open during downward travel and fluid is flushed through such channels and the restriction 20d to be certain that all of such openings are clean and free of debris prior to the jarring stroke.

When it is desired to initiate the jarring stroke with the jar I, the pipe string is pulled upwardly at the surface so as to exert a pull on the inner tubular body 10. Since the outer tubular body 12 is connected to the stuck fish, it, of course, does not move initially. As the inner tubular body begins to move, the engagement of the annular lip 30 with the inside surface of the restricted bore 12d provides sufficient frictional contact for preventing the piston 20 from initially moving upwardly with the bypass body 22. The body 22 is then moved upwardly relative to the piston 20 until the lower surface 20c seats on the annular seal surface 27b of the seal ring 27. When this occurs l(FIG. 4), further upward movement of the by-pass body 22 and the ring 27 is transmitted to the piston 20 and all of the oil or other fluid above the piston 20 must pass through the channels 22a and then through the restricted passage or orifice 20d. Since the passage 20d is extremely small, the fluid pressure above the piston 20 becomes high and offers a restraint to the movement of the tubular body 10. Such restraint continues while pulling upwardly on the tubular body 10, and a stretch occurs in the pipe supporting the tubular body 10 while the fluid slowly leaks or passes through the opening 20dfrom the area in the annulus above the piston 20 to the area in the annulus therebelow. Such restraint and stretching continues until the piston lip 30 reaches the upper end of the restriction 12d and enters the enlarged bore 12e. When that occurs, there is a sudden release of the restraint since the oil or other liuid above the piston 20 may then ilow freely around the piston 20; the tubular body 10 and its supporting pipe string then move upwardly rapidly as the stretch in the tubing or pipe is released, causing the jarring surfaces 10b and 12b to contact each other with a jarring blow.

Such jarring blow constitutes an upward force which is transmitted to the outer tubular body 12 and thus to the stuck pipe or fish connected to the lower end of the tubular body 12. If such jarring blow does not immediately release the stuck fish, the pipe connected to the tubular body 10 is then slacked off or lowered so that the piston assembly P returns to its lowermost position (FIG. 1) for a subsequent jarring stroke.

As previously explained, during the downward travel of the piston assembly P, the piston 20 is in its uppermost position and a full flow of fluid from below the piston 20 through the by-pass channels 22a to the annulus thereabove is effected so that there is substantially no restraint on the piston assembly P or anything connected thereto as they are lowered with respect to the outer tubular body 12. Thus, in the preferred embodiment, there is a free closing of the jar J on the downstroke so that the closing can be accomplished rapidly, and with or without impact in the downward direction. The bypass of the liuid is accomplished without utilizing any springs or other mechanism which may become fouled or malfunction during use.

Since the liuid chamber 14 is sealed olf by the upper and lower seals 15 and 16, respectively, the external well pressure is not additive to-the pressure created in the tool by the pulling action with the jar, but instead, the 'well pressure counterbalances some or all of the internal pressure developed in the tool during the pull stroke. Thus, in deep wells, the pressure of the well fluid may be 20,000 p.s.i. or even higher. In the typical prior oating piston construction wherein the iluid pressures externally and internally of the jar are the same prior to the pulling stroke, the pressure developed internally by the movement of the piston during the pull stroke is not counterbalanced by the external pressure so that the external pressure is of no value in preventing a bursting of the tool. With the present invention, since the external pressure is sealed olf by the seals 15 and 16 and the construction of the piston is such that it does not become jammed or stuck due to the compressive or squeezing action 'by the higher pressures on the tubular body 12, greater jarring forces may be obtained at the deeped well locations where the higher pressures exist and without bursting the tool. Thus, compared to the prior art, the jar 20 I of the present invention is capable of delivering greater jarring blows in deep wells than was heretofore possible.

pared to the by-pass body 22 of the piston assembly P.

Otherwise, the piston assemblies P and P-1 are essentially the same and like parts bear like reference numerals and/ or letters in FIGS. 3-5 and FIGS. 6-8.

The by-pass body 122 is cylindrical and is conned between the upper nut or sleeve 25 and the seal body 27 by a lower nut or sleeve 26 so as to be held securely on the inner tubular body 10. The external surface of the by-pass body `122 is formed with an upper substantially cylindrical external surface 122b which is of an increased diameter as compared to a lower substantially cylindrical external surface 122C. By reason of the reduction in diameter as compared to such external surfaces 122b and 122C, an annular stop surface or shoulder 122d is provided for engagement by the internal annular shoulder 20a of the annular piston 20. The diameter of the external upper surface 122b is less than the internal diameter of the inner surface of 4the annular lip 30 so that iiuid may flow therebetween during the return stroke of the piston assembly P-1, and also during the pull or jarring stroke. A plurality of longitudinal slots 122g are formed in the external surface of the by-pass body`122, and they are all in communication with each other through an annular recess 122e which is formed just above the lower external surface 122e.

The external surface 122e has a diameter which is slightly less than the internal diameter of the bore 20e of the piston 20. In the preferred embodiment, there would be a clearance of about 0.006 inch between the external surface `122C and the internal bore or surface 20e. Such clearance is suiciently large to permit substantially unrestricted flow of uid during the pull or jarring stroke as the fluid flows from above the piston 20 to the area therebelow, while at the same time preventing the passage of any metal particles or other foreign particles between the surfaces 122C and 20e. In fact, with the form of the invention illustrated in FIGS. 6-8, any foreign particles which may be in the oil or other uid in the chamber 14 would be -trapped or caught in one of the longitudinal slots 122a since such slots 122a in effect form pockets for such purpose. Thus, assuming a metal particle was present in the oil in the chamber 14 during the pulling stroke and it found its way downwardly below the lip 30, it could -then flow with liquid through the annular channel 122e to the first slot 122a in its direction of flow. The particle would be caught at the bottom of such slot 122:1 since the slots 122a do not extend downwardly to the lower end of the by-pass body 122, but instead terminate above such lower end. The relatively small clearance of approximately 0.003 inch between the surfaces 122c and 20e is not sufficient for particles of any detrimental size to pass therebetween, and therefore, such particles are trapped in one of the slots 122a and are prevented from passing out through the relatively small orifice or restriction 20d where they might otherwise obstruct or interfere with the flow of oil, or prevent an adequate closing of the body 20 with the seal Ebody 27. The depth of the orifice or opening 20d may vary, but typically, it would be approximately 0.008 to 0.016 inch deep, depending upon the size of the tool.

The restricted passage 20d thus still provides the restriction for the passage of iluid from above the piston 20 to therebelow during the pull or jarring stroke since the clearance of about 0.003 inch extends for the full circumference of the surfaces 20e and 122C and thus is much larger in area than the area of the passage 20d. However, it should be understood that the relative dimensions could be reversed so that a greater volume of uid could pass through the passage 20d than between the surfaces 20e and 122e, whereby the clearance between the surfaces 20e and 122e would provide the restriction for lluid flow during the pull or jarring stroke.

To facilitate the passage of the liquid to the orifice or restricted opening 20d, the lower edge 122i of the by-pass body 122 is beveled or chamfered throughout the full circumference of the lower edge of the body 122. It is to be noted that the chamfered edge 122f is below the lower end or bottom of each of the slots 12211 so as to provide the substantially cylindrical annular surface with a relatively small clearance between the surfaces 122C and 20e between the chamfered edge 122]C and the lower end of the slots l12211.

The operation and use of the modified piston assembly P-1 is essentially the same as heretofore described in connection with the form of the invention illustrated in FIGS. 1-5, the main difference being in the trapping of any foreign particles in the liquid or oil within the chamber 14 during the operation. This feature is, however, important because it prevents a failure of operation, or improper operation, of the apparatus which might otherwise occur if the piston 20 does not properly seat on the seal body 27 during the pulling or jarring stroke. Such might occur, for example, if a piece of metal became lodged in the orifice 20d and then became pressed so as to restrict the flow `through the orice 20d or so as to cause the lower end of the piston 20 to be spaced a distance above the seal body 27 greater than the orifice opening 20d. In either event, the apparatus would not function properly. Such a possibility is prevented by the trapping of the particles in the longitudinal slots or pockets 12241 in FIGS. 6-8, as previously explained.

The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape, and materials as well as in the details of the illustrated construction may be made within the scope of the appended claims without departing from the spirit of the invention.

What is claimed is:

1. A hydraulic jar, comprising an outer tubular body adapted to be connected to a stuck fish in a well, an inner tubular body telescoped within said outer tubular body and longitudinally movable relative thereto, jarring surfaces on said bodies for jarring contact with each other, a fluid chamber formed between said inner and outer tubular bodies, a restricted bore provided in said outer tubular body and disposed within said fluid chamber, the improvement residing in a piston assembly on said inner tubular body adapted to be moved in said restriction to develop a high pull on said inner pipe and to then move into a larger portion of said bore of the outer pipe for causing said jarring surfaces to contact each other with a jarring blow which is imparted to the stuck sh connected to the outer tubular body, the improved piston assembly comprising:

(a) a substantially annular piston having an upper lip with an outside diameter substantially the same 8 as the inside diameter of said restricted bore and with a flexible metal-to-metal sealing contact therebetween;

(b) a by-pass body on said inner tubular body and upon which said piston is mounted for limited movement of said piston relative thereto;

(c) said piston being movable upwardly relative to said inner tubular body to a position for freely bypassing fluid around said piston during the return stroke of the jar due to contact between said lip and said restricted bore;

(d) said piston being movable downwardly relative to said inner tubular body to a pulling or jarring position due to contact between said lip and said restricted bore;

(e) means restricting fluid ow in said fluid chamber around said piston during the pulling or jarring stroke; and

(f) said lip being flared outwardly and being flexible for maintaining the metal-to-metal sealing contact with said restricted bore.

2. The structure set forth in claim 1, including:

(a) seals on said inner tubular body and said outer tubular body above and below said restricted bore and said piston assembly to define said fluid chamber; and

(b) said seals isolating the fluid chamber in the annulus between said inner and outer tubular bodies which is substantially unaffected by change in the fluid pressure of the well fluids in which the jar is operated.

3. The structure set forth in claim 1, wherein:

(a) said by-pass body has an upper stop means therewith adapted to be engaged by said piston as said piston is moved downwardly in said restricted bore; and

(b) said by-pass body has a by-pass means from the upper end of said piston inwardly thereof to the lower end of said piston when said piston is in engagement with said upper stop means to provide for the unrestricted by-pass of fluid around said piston during the return movement of the piston.

4. The structure set forth in claim 1, wherein:

(a) said means restricting fluid flow around said piston includes a restricted passage formed as a part of said piston.

5. The structure set forth in claim 1, including:

(a) means for trapping small foreign particles in the uid as it flows around said piston during the jarring stroke.

6. The structure set forth in claim 1, wherein:

(a) said by-pass body has an upper stop means therewith adapted to be engaged by said piston as said piston is moved downwardly in said restricted bore;

(b) said by-pass body has a by-pass means from the upper end of said piston inwardly thereof to the lower end of said piston when said piston is in engagement with said upper stop means to provide for the unrestricted by-pass of fluid around said piston during the return movement of the piston;

(c) said means restricting fluid flow around said piston includes a restricted passage formed in said piston; and

(d) said by-pass body has a lower stop surface engageable by said piston during the pulling stroke for forming a seal therebetween so that fluid above the piston is forced through the restricted passage into the area below the piston to provide a restraint on the upward movement of the inner tubular body until said piston has moved above said restricted bore in said outer tubular body.

7. A piston assembly for a hydraulic jar, comprising:

(a) a by-pass body;

(b) a substantially annular piston mounted on said by-pass body for limited longitudinal movement relative thereto;

(c) said piston having a substantially cylindrical body with an upper outwardly flared flexible metal lip;

(d) means for restricting uid flow around said piston and through which fluid may be forced when said piston is at its lower limit of longitudinal movement relative to said by-pass body; and

(e) means between said piston and said by-pass body through which uid may freely flow to bypass said piston when said piston is at its upper position relative to said by-pass body.

8. The structure set forth in claim 7, including:

(a) co-acting stop surfaces on said piston and said by-pass body for limiting the longitudinal movement of said piston relative to said by-pass body so as to stop same at said lower limit and said upper position,

UNITED STATES PATENTS 2,988,147 6/1961 Webb 175-297X 3,088,533 5/1963 Sutliff 175-297 3,251,426 5/1966 LeBourg 175-297 3,302,736 2/1967 Kisling 175-297 3,399,741 9/1968 Monroe 175--297 3,405,773 10/ 1968 Sutlii 175-297 15 NILE C, BYERS, JR., Primary Examiner 

